Colorimetric determination of sulphur dioxide



ram-W .are well known. .on the proper maximum limit of allowableconcentration gases, such as sulfur dioxide-containing air. object ofthis invention is to provide color-changing'gels COLORIMETRICDETERMINATION F SULPHUR 1 a ,moxnm, l

. Gordon D. fatters on, Jr., Kenmore, N. Y., and Melvin Guy Mellon, WestLafayette, Ind.

No Drawing. Application March 16, 1953, o mium-342,754,;

" This inventionrelates to the colorimetric determination of sulfurdioxide. ,latesto the qualitative and quantitative analysis of air Moreparticularly, this invention recontaining sulfur dioxide by means ofcolor-changing gels.- -.i.The novel color-changing gels in thisinvention and the method of using these gels have particular utility in.the determination of sulfur dioxide in air in concentrations from 5 to1000 p. p. m. (parts per million, by

volume).

The problems of sulfur dioxide contamination in air For obvious reasons,exact agreement of sulfur dioxide has not been reached. However, theusually accepted maximum concentration for prolonged exposure is p. p.m. The tolerance for 30 to 60 minutes is generally given as 50 to 100 p.p. m., while 400 to.500 p. p. m. is dangerous even for short exposures.

Concentrations of the order of 1000 to 2000 p. p. m. i might be fatal.

, Most currently-used methods for sulfur dioxide in air depend onbubbling the sample through a solution and measuring the resultingchange in some chemical or physical property of the solution (or in somecases, of the Both the acidic and reducing powers of sulfur dioxide areused, and many methods and modifications are in theliterature. Nearlyall of them require bulky non-portable apparatusor a well-equippedlaboratory.

Itis therefore the principal object of this. invention to provide anovel means and method for: detecting sulfur dioxide and determining itsconcentration in a mixture of A further which are sensitive to sulfurdioxide concentrations ranging from 5 to 1000 p. p. m. A still furtherobject of this invention is to provide color-changing gels of thecharacter described whichnare adaptable for use in the field analysis ofthe sulfur dioxide content for air samples and United States Patent Oother gaseous mixtures from the standpoints of both 5 t loss insensitivity of the resulting gel.

plete removal of water by evacuation to low pressures sensitivity.

as ethyl silicate, (EtO)4Si, etc. can be used, while other T vanadatesalts such as the alkali metal vanadates like sodium vanadate can besubstituted for ammonium vanadate.

In practlcing this invention, the gel-able siliceous compound can becontacted with an ionic solution of the Y vanadate salt to form thesulfur dioxide-sensitive gel. For example, excellent results areobtained'by impregnating activated silica gel in granular form withan-aqueous solution of the vanadate salt, and removing the exoesssolution by evaporation or filtration. Passing a. sulfur concentration.

2,785,959 M Patented Mar. 19, 957

dioxide-containing gas through the impregnated granules thus obtainedproduces color changes which can be used for the colorimetricdetermination of the sulfur dioxide Ice The vanadate impregnated silicagel granules are preferably used by packing them into a section of atube having a transparent wall for viewing the packed section.

'For example, an 8 to 10 mm. long section of a glass tube having aninternaldiameter of from 2 to 4 mm. can be used. The bed of granules canbe held within the packed section of the tube by, suitable porousretaining members, such as plugs of loosely packed glass wool or -glasscloth, etc. A measured sample of the gaseous mix- -ture to be tested'(e. g., a 50 to ml. air sample) is then passed through the tube,preferably at a uniform slow rate. The colors produced usually divideintofive sharply defined chromatographic bands ranging in color fromblue to yellow. Excellent sensitivity is obtained within the range from5 to 1000 p. p. m. of sulfur dioxide,

anddeterminations can also be made outside of this range, if desired.Various methods can be used to determine the sulfur dioxideconcentrations from the color changes of the gel, but all methodsinvolve essentially a comparison of the color changes with a standardset of colors for known sulfur dioxide concentrations under comparableconditions. One method (preferred for higher concentrations) depends onmeasurement of the length of the section of the gel which becomesdiscolored by the gas. The other (preferred for low concentrations)depends on a colorimetric matchof the entire column of 'the gel with astandard series of colors, each ofw'hich corresponds to a differentconcentration. The matching may be done either visually orphotoelectrically, and a number of instruments have been developed forsemiautomatic photoelectric determinations using gels. Alternatively,the volume offthe gas sample required to produce desired color changesor a desired length of the color column can be observed. V

- The exact amount of ammonium vanadate usedis not critical.Howeven'there are approximate minimum and maximum limits which shouldnot be exceeded. ratio of vanadium to silicon is too low, the colorsbefore and after exposure to sulfur dioxide are too dilute or vague. Toohigh a ratio causessolid salts to form separately from the gel granules,and the result is a hetero- If the geneous mixture whose color cannot bematched or described conveniently. The best quantities are found to beapproximately 2 ml. of silica gel granules per 20 ml.

of saturated vanadate solution (aqueous). 60

The vanadate-silica gels can be prepared by immersing the activated gelgranules in a solutionof the vanadateyfollowed'by removal of thesolvent. The preferred method of removing the solvent is to evaporatesubstantially completely the water from the gel-solution mixture byheating. Decantation or filtration may result in some or by desiccationmay also reduce or not improve the Alternatively, the silica gelgranules can be coated with ammonium vanadate by placing the activatedgel in a g'lasstube and adding saturated ammonium vanadate'solutiondropwise at the same time that a heated, dry flow of airispassed upward from below. The rate of addition of the solution should becoordinated with the rate of flow of-the drying air, so that the gelgranules remain free-flowing and dance around inside the glass tubeuntil all the solution has been added. Gels prepared by this method ,areequally or more sensitive to low concentrations'of sulfur dioxide,compared with other vanadate silica gels. They are also somewhat moreuniform HICOIOX'.

However, com- 1 can also be prepared using ethyl silicate, EIOMSL'as'the source of silicon instead of silica gel. For example, 10 ml. ofsaturated ammonium vanadate can be stirred with ml. of:e thylsilicate,(boilingra1g -167-f- -l69 which i is? then. gelled and formedintof yellow solid granules.

There are a number of points in the fi rst -method-eof though the exacttemperatureisnot -critical,-;optimum results are obtained withevaporationand-dryingat 50 lgnition of thezisilica at about'600?,;followed by 3i 'Ihe-pH-ofthe vanadate treating solution is"important droxide solutioncan be used to raise the pH if desired.

The sensitivity of the color-changing materialis-somewhat improved byhaving an' ammonium salt present in the "treating solution,-fthat is,an-ammonium salt other than. or in addition to-ammoniumwanadate.For-example, small amounts-of ammonium nitrate, ammonium chloride, orammonium sulphate canbeused.

'The particle size of the gel is of importance, although not critical.Small particle-size powders havea tendency granules down to about 2O0mesh size -givesatisfactory results. Optimummesh-sizeis-25-35. v

Instead of silica gel or other-,gel-contaiuing silicon, other sulfurdioxide-absorptive gels can be used; such as aluminagel, titania gel,etc. -However,-superior'result s are I achievedlby employing gelscontaining- -siliconwhich is available, presumably to form heteropolycompounds 'with the vanadium in the vanadate salt. deuce to indicatethat a combination form's betweemthe vanadate ionand the silica gelwhich resultsinauniquely su' lfur dioxide-sensitive material.combination is probably a heteropoly compound containing silicon-=as"the central atom and'vanadium as thecoordinate atom. The actualreactions which cause the color changeshjave not.been conclusivelydetermined, but may conceivably be due to' either (oi-both.) the acidicor reducingproperties of the isulfur' dioxide. The appearance of; fivedistinct color bands would seem to .indicatefiveldefinite stepsfin thetotal reaction, which might coirespond-to-fivudiffer'ent oxidationstates.

The 'cjoir be used for the. chromatographic determination of other aeidifgases, suchas hydrogen sulfide, hydrogen-chloride, etc. "Hbweverfthe;colo'rspi'oducedby either-hydrogen sulfide orhydrogen chloride, forexamplegarelquite dis- 'tiuctfronithe colorsp'roducd by sulfurdioxide,-;and-n;uch larger amounts, of these; gases arqnecessary tocause color changesithanof sulfur dioxideg Hence they{dofliotlin-"ter'tere seriously when present in -a sample beinganalyzed Q'r' Qa: 7

-There -'is evijis nverition -is further. illustrated, bythe follovvin gpr eparation described abovewhere ;-the temperature is animportantfactor. 9f: these, the most importantqare the temperatures ofevaporation and otifinaLdry-ing. Al-

to 7'0 C. ZProlonged heating, is undesirable. ,aHeating the solutionbefore addition of the--silica ;gel= is-not--bene- "ffic'al. cooling ina desiccatonimmediately; prior-to the addition lot the ammonium vanadate-solution, 'resultsin, somewhat .lbettercolors upon exposure to sulfurdioxide.

to cause streaming and streaks to appear. However,

changing material Quins invention can s ture was filtered and the solidwas dried at 110. C. -The vanadate-silica gel product thus obtained waspacked into a number of small glass -tubes, and sulfur dioxide waspassed through some of the tubes. The yellow product gave an immediatecolor change with sulfur dioxide from yellow to green and then to blue.Later similar preparations with thesulfur dioxide passed through thetube more s1dw1y--s11ewed a color front bf bi'dv'v'n "Which"'1fiovedslowly from the entrance enact the tube. When the passage of sulfurdioxide was "stopped- "before complete saturation, the tube had a'series ofchromatographic-like bands, being yellow at the unreactedendand progressing through" brown "and" a brownish-green] to a 'light" blueat the inlet' end. The colors'were stable for at least ten day's.

Example II A seriesbbgels were prepared 'by heatingdwo' ml. of silicagel granules with 25' 1 ml'. of satu'rated ammonium vanadate -solutionat different temperatures for diifeient lengthsof time"without'cotnplete evaporation. -Theex cessliquid then'wasremoved' bydecantation or by filtration. The more thoroughly the solutionadheringto-the -gel.was removed ('by" washing, by absorption-with filterpaper; or bysuction-filtration), the-less yellow the resulting gel wouldbe and the less sensitive it Would-be tolo'w concentrations. Thisappears to indicate that physical dep osition of the animonium vanadatesalt was necessary -and that adsorption of thev'anadateby thesilicasurface 'Wasnot whollyrespons'ible for the formation of thecolbrchanging product. 7 7

' Ex'dmple I III 1A1 ditfer'e'ntmethod for coating silica'gel granules"with -smmoniumvanadate was found. It consisted of placing the activated'gel in a glass tube and adding saturated ammo'nium vanadate "solution'dropwise 'from above at the same time that a heated, dry'flow of'airwas' 'passedin from below. The 'rate' of addition of the solution was-coordinated with'the rate of flow of the' drying air,"'so

'th'aitfthe g elfgranules remained substantiallyfree-flowing and hencedanced around inside theglassftubeuntil an-er the ammonium vanadate'solution was added. The air"-was dri'edin somecases bybubbling throughconcenftra'tedi sulfuric" acid'o'r throughdr'ierite. Itfwas slightlyWarmed'by' further assa e-throngs; msseaii immersed in boiling water.Gels prepared" by this method were A series "of observations was made onthe -'-colored baiids' caused by a mixture of air and SOa on 'vanadate-"sili'calgels, prepared as descri-bed in- Example-I. For theseexperiments, a sufiicient amount of the gel was placed in a-jtube toform a column two or three: inches long. Then-the gas 'or vaporwas'passed'into *theicolumn until the --front of the colored area hadtraversed about three-fourths of the "length of the'tube;

, b V heating "zoo-"mesa sill afjgel 'w 1h a saturated. solution of amonimnvanadatei "After" needing ever'rfight' "Repeatedexperiments withsulfur dioxide on' the vanada'te-siliea-gelsinvariably produced fivechromatographic- 1 (a) 4 Hen-m blue .(lSic).

(d) olivezgreen (2415ni).

Bu'tteryellow-sunlight yellow le ala).

'i ja The borders between these areas were quite sharp and distinct ineach case and the colors formed were found to hei-pern'lanent in tubeswhich were'afterward open to the room air and exposed to sunlight forperiods of several months. or *lo'ngen -The fivebands were not of equallength. The brown band between the green and blue areas generally didnot appear immediately and would show up about 20 to 30 minutes afterthe tube had been exposed. It was also always very narrow compared totheother color-bands.

' Example V For example, ten ml. of saturated ammonium vanadate werestirred with five ml. of ethyl silicate (boiling range 167-169) in thecovered 50-ml. beaker. The ethyl silicate layer became yellowimmediately, and the Example VI A further experiment was made followingsubstantially the procedure of Example I, except that sodium vanadatewas substituted for the ammonium vanadate in the treating solution. Theresulting vanadate-silica gel composition was found to be sensitive tosulfur dioxide and to produce similar color changes to the ammoniumvanadate-treated silica gels.

Example VII For preparation of a gel color-sensitive to sulfur dioxide,the following typical procedure is applicable. First obtain or prepare apure granular silica gel (preferably, although not necessarily, similarto that described by G. M. Shepherd in U. S. Patent 2,487,077) andfurther activate it by heating to a low red heat (e. g., 600 C.),followed by a cooling in a relatively dry atmosphere, such as in adesiccator containing calcium sulfate or other desiccant. Then measureout a volume of the gel (e. g., ml.), place it in a glass vessel and adda volume of saturated aqueous solution of C. P. ammonium vanadate whichis ten times the volume of the gel (e. g., 100 1111.). Mix thegranule-solution mixture gently to free the bubbles of gas which form onthe gel surface and obtain complete contact between gel-particlesurfaces and the liquid. Heat the mixture in a ventilated hot air ovento a temperature of C. (plus or minus 10) until the water has evaporatedcompletely, leaving a freely-flowing bright yellow granular gel. Thisgel is now ready for use and may be stored in a stoppered bottle orplaced in short lengths of four to eight mm. transparent glass tubing.This is conveniently done by placing a small wad of glass wool insidesuch a tube near one end. Then add a measured amount of the sensitivegel and confine it with another porous plug of glass wool.

The tube is then taken to the location of the gas to be analyzed,connected to any of several means of drawing a volume of air through thetube at a slow, relativelyeonstant, rate. Any one of a number ofcommerciallyavailable metal holders with clamps to mount the tube may beused. The tube is so mounted and connected to a rubber bulb having aknown volume of 25 to 50 ml. which may be squeezed to expel] its air tothe atmosphere and then refill at a constant slow rate by drawing thegas sample through the glass tube containing. the gel. A

probe tube may be connected to the intake end of the gel-containingglass tube if samples froin a distance are desired. Repeated squeezingof the rubber bulb maybe desirable for large samples and/or for'flushingthe latent air contained in theprobe tubing. The color changewhich occurs on the gel may be observed visually and comparedwith othergels similarly exposed to known concentrations or with standardcolor-cards having printed areas of various colors corresponding todifferent amounts of sulfur dioxide.

Alternatively, the glass tube may be mounted so that a light beam hitsthe gel and then strikes a photocell which is connected electrically toa meter which indicates the amount of light passing through, orreflected from, the gel. A decrease in the amount of light is indicatedby the reading of the photometer and the change in the meter reading isthen a measure of the amount of sulfur dioxide causing the color change.

Unlike other gels of different compositions which are used to detect anddetermine other gases (e. g., carbon monoxide), no guard gel isnecessary to remove possible interfering constituents of the air sample.In ordinary atmospheric samples the other commonly occurring minorcomponents such as water vapor, carbon monoxide, carbon dioxide, therare gases, etc., neither cause color changes nor interfere with thesensitivity of this gel to sulfur dioxide. In fact, the use of a guardgel is generally undesirable since most of the possible gels wouldremove the sulfur dioxide before the sample reaches the sensitive gel.

The utility of the color changing gels of this invention.

is not limited to the analysis of gaseous samples, although its greatestutility is for this use. However, the desired color changes do occur inaqueous solutions containing sulfite ions (SO3=). Therefore, it will beunderstood that the color changing gels of this invention are alsosusceptible to the measurement of the sulphur dioxide concentration inan aqueous solution of sulphur dioxide.

While in the foregoing specification this invention has been describedwith particular reference to certain preferred embodiments thereof andspecific details and examples have been given by way of illustration, itwill be apparent to those skilled in the art that the preferredembodiments, specific details, and examples can be varied considerablywithout departing from the spirit of the invention.

We claim:

1. The colorimetric method of determining the concentration of sulphurdioxide in a fluid medium, comprising contacting a sample of said mediumwith a colorchanging material sensitive to sulphur dioxide to obtaincolor changes in said material, and comparing the color changes thusproduced with a standard set of color changes for known sulphur dioxideconcentrations under comparable conditions, said color-changing materialbeing in granular form and comprising a siliceous gel impregnated with avanadium salt selected from the group consisting of ammonium vanadateand the alkali metal vanadates, said color-changing material beingsubstantially free of said vanadate salt in solid form separate from thesiliceous gel granules while containing a suflicient concentration ofsaid vanadate salt to produce distinct color bands on exposure of saidmaterial to sulphur dioxide.

2. The method of claim 1 in which said fluid medium is air, saidsiliceous gel is activated silica gel in granular form, andsaid vanadatesalt is ammonium vanadate.

References Cited in the file of this patent UNITED STATES PATENTS2,027,316 Johnson Jan. 7, 1936 2,174,349 Littlefield Sept. 26, 19392,271,618 Block et a1. Feb. 3, 1942 (Other references on following page)

1. THE COLORIMETRIC METHOD OF DETERMINING THE CONCENTRATAION OF SULPHURDIOXIDE IN A FLUID MEDIUM, COMPRISING CONTACTING A SAMPLE OF SAID MEDIUMWITH A COLORCHANGING MATERIAL SENSITIVE TO SULPHUR DIOXIDE TO OBTAINCOLOR CHANGES IN SAID MATERIAL, AND COMPARING THE COLOR CHANGES THUSPRODUCED WITH A STANDARD SET OF COLOR CHANGES FOR KNOWN SULPHUR DIOXIDECONCENTRATIONS UNDER COMPARABLE CONDITIONS, SAID COLOR-CHANGING MATERIALBEING IN GRANULAR FORM AND COMPRISING A SILICEOUS GEL IMPREGNATED WITH AVANADIUM SALT SELECTED FROM THE GROUP CONSISTING OF AMMONIUM VANADATEAND THE ALKALI METAL VANADATES, SAID COLOR-CHANGING MATERIAL BEINGSUBSTANTIALLY FREE OF SAID VANADATE SALT IN SOLID FORM SEPARATE FROM THESILICEOUS GEL GRANULES WHILE CONTAINING A SUFFICIENT CONCENTRATION OFSAID VANDATE SALT TO PRODUCE DISTINCT COLOR BANDS ON EXPOSURE OF SAIDMATERIAL TO SULPHUR DIOXIDE.